The application of coating materials to objects has conventionally been accomplished in spray booths having an elongated tunnel-like construction formed with an inlet for the ingress of the object, a coating application area, a curing or drying area in some designs and an outlet for the egress of the object. In some systems, "conditioned" air, i.e., humidified and filtered air, is introduced by a blower or feed fan into a plenum chamber at the top cf the spray booth, and directed downwardly toward the floor of the booth. Alternatively, negative pressure is created within the booth interior which draws ambient air downwardly from the top of the booth to the floor. In either design, oversprayed coating material is entrained within this downward flow of air in the booth interior and delivered to a powder collection and recovery system at the base or sides of the booth where filters capture the oversprayed coating material and exhaust filtered or clean air to the atmosphere or back to the system for reuse.
In coating large objects such as vehicle bodies, both vertically and horizontally oriented surfaces must be coated as the object moves through the booth interior. For example, the fenders and doors of a vehicle body travel vertically through the spray booth, while the hood, roof and trunk areas of the vehicle body are oriented horizontally in the course of movement through the booth. In order to obtain a uniform coating on all of these surfaces, it has been the practice to provide one or more coating dispensers at the side walls of the spray booth to coat the vertically oriented surfaces of the vehicle body, and to mount one or more coating dispensers overhead near the top of the booth to dispense coating material downwardly onto the roof, hood and trunk areas of the vehicle body. Depending upon the size of the vehicle body, the number and location of coating dispensers utilized and a variety of other factors, mounting structure is needed to move both the side and overhead coating dispensers relative to the vehicle body in order to obtain the desired film build-up on all surfaces. In many instances, deposition of the coating material on the object is further enhanced by electrostatically charging the coating material as it is discharged from the dispensers and maintaining the vehicle bodies or other objects to be coated at a different or ground potential so that the charged coating material is electrostatically attracted to the objects.
One problem associated with the structure commonly employed for moving coating dispensers within the interior of spray booths involves the potential for contamination of the coating material as it is applied to the vehicle body or other object within the booth. In the past, the mechanisms for moving the side mounted and overhead mounted coating dispensers have been exposed within the spray booth interior. This can create contamination of the coating material as it contacts the lubricated connections and drive structures associated with the dispenser moving devices. Additionally, the lines which supply coating material to the dispensers, and, in some designs, the lines which carry high voltage electrostatic cables to the dispensers, are also commonly exposed within the interior of the booth. In powder applications, these utility lines can create further contamination of the coating material by abrasion and fall-out of non-atomized powder onto the substrate.
Another problem with devices currently utilized to move coating dispensers involves the creation of air turbulence within the booth interior. Turbulence is of particular concern when powder coating material is utilized as a replacement for high solids, liquid paint material. Powdered resin coating material has a much lower density and is much lighter in weight than atomized liquid paint particles, and is discharged from a dispenser in a cloud-like pattern onto the object to be coated. It has been found that in applying low density, lightweight powder material onto a vehicle body, and particularly its horizontal surfaces such as the hood, roof and trunk, systems for moving the coating dispensers up and down, side-to-side and/or pivotally with respect to such surfaces can result in the creation of turbulence within the booth interior. That is, the downward flow of air within the booth interior which entrains oversprayed coating material becomes turbulent as the overhead coating dispensers are moved with respect to the object to be coated. This turbulence can create an inconsistent film build-up on the object because it disrupts the flow of powder material between the coating dispensers and the object to be coated.